1. Field of the Invention
The present invention relates to golf balls. Specifically, the present invention relates to improvement of density distributions of golf balls.
2. Description of the Related Art
A golf ball hit with a golf club flies out with a launch angle relative to the horizontal direction. The launch angle is caused by the fact that the head of the golf club has a loft angle. At the time of flying out, the golf ball has so-called backspin. The backspin is caused by a shear force generated when the golf ball collides against the head having the loft angle.
The shear force is applied to the surface of the golf ball. Since the golf ball is an elastic body, even when the shear force is applied to the surface, the center of the golf ball attempts to remain still due to a moment of inertia. Therefore, due to the shear force, the time at which rotation of the center of the golf ball begins is slightly delayed with respect to the time at which rotation of the surface of the golf ball begins. Due to this delay, torsional strain occurs within the golf ball. When the torsional strain is eliminated, a negative shear force is applied to the golf ball. The direction of the negative shear force is a direction in which overspin is provided to the golf ball. At the time of impact, a positive shear force and a negative shear force are applied to the golf ball. In a correctly-hit golf ball, the positive shear force is greater than the negative shear force. The negative shear force does not exceed the positive shear force. Therefore, to the correctly-hit golf ball, backspin is provided, not overspin.
The trajectory of a golf ball after launch is greatly influenced by the launch angle and the backspin rate. A golf ball having a high launch angle tends to have a high trajectory. On the other hand, a golf ball having a low launch angle tends to have a low trajectory. By backspin, a lift force is generated in a golf ball. A golf ball having high backspin tends to have a high trajectory. A golf ball having low backspin tends to have a low trajectory.
Golf players' foremost requirement for golf balls is flight distance. In particular, golf players place importance on flight distances upon shots with a driver and a long iron. In order to achieve a large flight distance, an appropriate trajectory height is required.
In a golf ball that achieves a high trajectory by a high spin rate, a flight distance is insufficient. One of the reasons is inferred to be that the higher the spin rate is, the greater the drag is generated. Another reason is inferred to be that a lift force is applied perpendicularly to the flying direction and thus a force to pull the golf ball backwards is generated by the lift force until the highest point of the trajectory.
Meanwhile, in a golf ball that achieves a high trajectory by a high launch angle, a large flight distance is obtained. A golf ball is desired which has a low backspin rate and a high launch angle when being hit with a driver or a long iron.
Golf players also place importance on spin performance of golf balls. When a backspin rate is high, the run is short. It is easy for golf players to cause a golf ball, to which backspin is easily provided, to stop at a target point. When a sidespin rate is high, the golf ball easily curves. It is easy for golf players to intentionally cause a golf ball, to which sidespin is easily provided, to curve. A golf ball to which spin is easily provided has excellent controllability. In particular, advanced golf players place importance on controllability upon a shot with a short iron.
There have been various proposals for the density distributions of golf balls. JP2002-331047 (U.S. Pat. No. 6,533,682) discloses: a golf ball that includes a high-specific-gravity core, a low-specific-gravity mantle layer positioned outside the core, and a low-specific-gravity cover positioned outside the mantle layer; and a golf ball that includes a low-specific-gravity core, a high-specific-gravity mantle layer positioned outside the core, and a cover positioned outside the mantle layer. Similar golf balls are also disclosed in JP2002-325863 (U.S. Pat. No. 6,494,795), JP2005-111246 (U.S. Pat. No. 6,786,838), and JP2008-6302. Any of these publications discloses a technique to adjust the moment of inertia of the golf ball. For example, paragraph [0007] of JP 2002-325863 states that “Specifically, if the density of the ball is shifted or distributed toward the center of the ball, the moment of inertia is reduced, and the initial spin rate of the ball as it leaves the golf club would increase due to the lower resistance from the ball's moment of inertia. Conversely, if the density is shifted or distributed toward the outer cover, the moment of inertia is increased, and the initial spin rate of the ball as it leaves the golf club would decrease due to the higher resistance from the ball's moment of inertia.” This publication states that if the density of the ball is shifted toward the center of the ball, the moment of inertia is reduced and the initial spin increases. JP2002-331047 has a similar description at paragraph [0008] but has a misdescription regarding increase/decrease of spin.
JP2011-172929 (U.S.2011/0207555) discloses a golf ball that includes a core, a mid layer, a cover, and a high-specific-gravity member. The high-specific-gravity member contributes to adjustment of the moment of inertia of the golf ball. A similar golf ball is also disclosed in JP2011-172930 (U.S.2011/0207554).
JP11-89969 discloses a golf ball that includes high-weight grains or ring. The grains or ring contributes to adjustment of the moment of inertia of the golf ball.
JP2009-160018 discloses a method of designing a golf ball. In this method, a golf ball whose elastic modulus distribution is made appropriate is obtained.
There have been also various proposals for the hardness distributions of golf balls. A golf ball having an outer-hard/inner-soft structure is commercially available. In the golf ball, the negative shear force is great. When the golf ball is hit, the golf ball flies with a high launch angle and a low spin rate. When the golf ball is hit with a long iron, a large flight distance is obtained.
As described above, a golf ball having an outer-hard/inner-soft structure has excellent flight performance. However, golf players desire further improvement of flight distance. An excessive hardness gradient impairs the durability of the golf ball. It is impossible to meet the golf player's requirement for flight distance only by an outer-hard/inner-soft structure.
When a golf ball having an outer-hard/inner-soft structure is hit with a short iron, the golf ball largely slips relative to the face of the golf club since the outer portion of the golf ball is hard. By this slip, spin is suppressed. The golf ball has inferior controllability upon a shot with a short iron.
An objective of the present invention is to provide a golf ball having excellent flight performance when being hit with a long iron and having excellent controllability when being hit with a short iron.